7. Abstract The field of developmental cancer therapeutics is at an exciting juncture. The major advances in the understanding of molecular defects leading to cancer progression and chemoresistance, together with the dramatic increase in the available molecular targeting drugs, provide unprecedented opportunities to develop potentially useful combination therapies. The scientific pursuit is further aided by the recent FDA decision to allow clinical testing of combinations involving more than one unapproved drugs. The newer agents target steps in signaling pathways that control gene expression, including inhibitors of binding of growth factors to their receptors and inhibitors of such as tyrosine kinase inhibitors. The more recent introduction of RNAi agents provides additional opportunity for gene silencing on the post- transcriptional level. The goal of this project is to develop the concept (mathematical framework) and methods for combining agents that modulate different steps in a signaling cascade, with the goal of achieving true synergy or clinical synergy. We propose three aims, using Wnt/?-catenin/survivin signaling as the model pathway. This pathway plays multiple important roles in cancer development and chemoresistance. The first aim is to establish interactive pharmacodynamic models (IPM) on combinations of agents targeting pre- and the post-transcriptional signaling events. These models are used with the single agent PD parameter values (slope, Emax, EC50, selected from literature values and from experimental data in Aim 3) to simulate the results of different combinations. The in silico results are analyzed with the method developed in Aim 2 to identify synergistic, additive or antagonistic combinations for experimental validation in Aim 3. The second aim is to develop a method that quantifies, with statistical certainty, the interactivity among drugs with parallel or non-parallel concentration-effect curves. The third aim is to obtain experimental validation of model predictions. The first objective is obtain in cultured cells the PD parameter values for single agents directed at several pre- and post-transcriptional steps in Wnt/?-catenin/survivin signaling (e.g., antibody against Frizzled, siRNAs (against Disheveled, PP2A, survivin), and a small molecule drug OHL (which was found to reduce ?-catenin/survivin signaling). The effectors include wild type survivin mRNA and protein levels, levels of total and the active non-phosphorylated ?-catenin, and Tcf/Lef transcriptional activity. The second objective is to evaluate if the model-predicted interactivity is achieved in vitro and in vivo. Potential gains are three-fold. First, the results will assist the development of combinations targeting multiple steps in a signaling pathway. Second, in view of the critical roles of the Wnt/?-catenin/survivin signaling in tumor development, the results will further yield insights on its modulations for therapeutic gains. Third, overcoming the survivin-mediated chemoresistance, a major cause of treatment failures, will improve the efficacy of chemotherapy.